"I didn't know where he got hit at first, his elbow or hand," D-backs manager A.J. Hinch said. "When I got out to first base, his hand was pretty swollen already. He's never going to come out of that game right then. When he went out to play catch, he couldn't put any pressure on his left hand inside his glove."
X-rays showed a fracture of the fifth metacarpal on the left hand, and Byrnes will require surgery.
"You never like to see anybody put out of the game like that," Hinch said. "He brings a lot of energy and he's been grinding and battling himself and he cares a lot, he's been wanting to help turn this around. It's another punch in the gut as we continue to take some blows. Again it's going to be an opportunity for somebody to get more playing time. It definitely is unfortunate to lose Eric."
Chris Young, who has been battling a sore groin, will likely be back in the lineup on Friday. Young, Gerardo Parra and Justin Upton will be the team's primary outfielders until Byrnes returns.
Hinch said the team would decide late Thursday night what roster move to make. One likely scenario is calling up Triple-A outfielder Alex Romero.
Byrnes, who missed most of last season with hamstring issues, is no stranger to the injury bug. He was hitting .216 with five homers and 24 RBIs in 194 at-bats this year.
"That's the way it's been going around here," infielder Mark Reynolds said. "There are a lot of guys hurt. The pitch squared him up on his hand pretty good. It's a tough break, but he's resilient and he's come back from a hamstring off the bone, so I'm sure he'll get this healed up and he'll be back as soon as he can."
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.